Builds a complete, usable map from the string passed in seed. Each triplet
composed of seed/map_w/map_h should build the same map on any computer,
in any context, for a given version of the game (no backwards compatibility).

Return value: new map on success, NULL on failure.

Function: char *lw6gen_seed_new(lw6sys_context_t * sys_context)

sys_context: global system context

Generate a new random seed. The seed is composed of letters and numbers.